1. Field of the Invention
This invention relates to a method and apparatus for the removal of at least one condensable component from a process stream comprising the condensable component in a manner which allows the recovery of its latent heat of vaporization. This invention is applicable to flue gases from boilers or process heaters or to other process streams which contain significant quantities of a condensable component at a temperature above the dew point of the condensable component. More particularly, this invention relates to a method and apparatus in which a vapor-phase component of a process stream, such as water vapor in flue gases, can be returned to the process in liquid form, such as feed water for steam generation, which recovers the latent heat of vaporization.
2. Description of Prior Art
Many industrial processes produce process streams containing condensable components, such as water vapor and volatile organics. Typically, it is desirable to remove and recover these condensable components from the process streams for environmental and/or economic reasons. In addition, it is also desirable to recover the latent heat of vaporization associated with such condensable components as a means for reducing process energy requirements. The use of heat exchanger-based condensers for the recovery of condensable components of process streams and the latent heat of vaporization associated therewith is well known to those skilled in the art.
Methods and apparatuses for the selective removal of one or more components from a gaseous mixture are well known. U.S. Pat. No. 5,753,009 teaches a method and apparatus for selective removal of one or more components from a multi-component gas/vapor mixture by membrane fractionation. The membrane fractionation gas removal system comprises a feed chamber containing the gas/vapor mixture, at least one porous membrane having a first side which contacts the gas/vapor mixture in the feed chamber, at least one non-porous membrane having one side which contacts the second side of the porous membrane, which non-porous membrane has a permeability selective to one or more components of the gas/vapor mixture, an exit chamber connected to the second side of the non-porous membrane such that the component exiting the non-porous membrane enters the exit chamber, and an evacuation member connected to the exit chamber for evacuating one or more components from within the exit chamber. U.S. Pat. No. 4,875,908 teaches a process for selectively separating water vapor from a multi-component gaseous mixture in which the multi-component gaseous mixture comprising the water vapor is passed along and in contact with a membrane which is selectively permeable to water vapor. The use of membranes for selective removal of one or more components of a gaseous mixture is also taught by U.S. Pat. No. 4,583,996 (inorganic porous membrane), U.S. Pat. No. 3,980,605 (fibrous semi-permeable membrane) and U.S. Pat. No. 3,735,559 (sulfonated polyxylylene oxide membranes).
Methods and apparatuses for selective removal of water vapor from a gaseous mixture and condensing the separated water vapor to recover its latent heat of vaporization are also known. U.S. Pat. No. 5,236,474 and related European Patent Application 0 532 368 teach a process for removing and recovering a condensable vapor from a gas stream by a membrane contactor in which a gas stream containing a condensable vapor is circulated on one side of hollow fiber membranes while cool extraction fluid is circulated on the other side under a total pressure differential. As a result, the condensable vapor in the gas stream is condensed in the gas stream and the condensed vapor, i.e. liquid, permeates the membrane and becomes entrained in the cool extraction fluid.
U.S. Pat. No. 4,466,202 teaches a process for recovery and reuse of heat contained in the wet exhaust gases emanating from a solids dryer or liquor concentrator by preferentially passing the vapor through a semi-permeable membrane, compressing the water or solvent vapor, and subsequently condensing the water or soluble vapor in a heat exchanger, thereby permitting recovery of its latent heat of vaporization for reuse in the evaporation process. It will be apparent to those skilled in the art that a substantial amount of energy will be required to compress the water or solvent vapor in accordance with the process of this patent. U.S. Pat. No. 5,071,451 teaches a vapor recovery system and process that permits condenser vent gas to be recirculated. The system includes a small auxiliary membrane module or set of modules installed across a pump and condenser on the downstream side of a main membrane unit, which module takes as its feed the vent gas from the condenser and returns a vapor-enriched stream upstream of the pump and condenser.
Notwithstanding these and other known methods and apparatuses for latent heat recovery, it will be appreciated by those skilled in the art that only a small fraction of available waste energy is recovered, particularly in the case of conventional heat exchanger-based condensers.
Accordingly, it is one object of this invention to provide a method and apparatus for recovering condensable components from a process stream, said method having improved energy efficiency over conventional methods and apparatuses.
This and other objects of this invention are addressed by a method for removing a condensable component from a process stream comprising the condensable component in which the first side of a permselective membrane is contacted with the process stream comprising the condensable component, the condensable component is passed through the permselective membrane to a second side of the permselective membrane, forming a condensable permeate, and the condensable permeate is contacted directly with a liquid stream comprising a liquid form of the condensable permeate at a temperature below that of the process stream, resulting in formation of a condensed permeate. The condensed permeate may then be returned to the process by which the process stream was generated.
The method of this invention may be carried out in an apparatus comprising a process stream conduit and a condenser disposed within the process stream conduit. The condenser comprises at least one wall enclosing a condenser chamber and forming at least one liquid stream inlet and at least one condensed permeate outlet. The wall of the condenser comprises a permselective membrane suitable for selectively passing at least one condensable component of a process stream flowing through the process stream conduit from the process stream, through the permselective membrane, and into the condenser chamber.
The essence of this invention is the use of the permselective membrane, which is a membrane that allows one condensable component to pass through selectively, forming a permeate stream, while substantially preventing other components from passing through, separation of the permeate stream comprising the vaporized state of that condensable component from the process stream, and immediately condensing the permeate by direct contact with a flowing, sprayed, or atomized liquid stream, also referred to herein as the condensing liquid, comprising the liquid form of the same condensable component as the permeate. The temperature of the process stream and the mass flow and/or temperature of the condensing liquid must be selected or regulated such that contact of the condensable component vapor with the condensing liquid removes a sufficient amount of heat from the vapor to permit condensation of a major portion of the permeate vapor. The latent heat of vaporization of the condensed portion of the component that passes through the membrane is then transferred to the condensed liquid stream as sensible heat. In this manner, energy is transferred from the process stream to the incoming condensing liquid stream and returned to the process very efficiently. The driving force for this transfer of heat and mass is the differential partial pressure of the condensable component across the membrane.